For continuous determination of a level of material in containers, filled e.g. with liquids or with bulk material, mostly sensors are employed being based on radar technology, measuring the round trip time of microwaves from the sensor to the surface level of the material to be measured and back. Microwaves, lying in a frequency range from approximately 1 to 100 GHz, are radiated via antennas. The microwaves are reflected on a surface of the filling good and are received again by the antenna. Besides, devices are known in prior art, using a wave guide, to guide the wave from the sensor to the surface of a material to be measured. The reflection of the waves at the surface of the material is based on the change of the propagation impedance for the wave at this point.
For the determination of the round trip time of a wave, different radar techniques are known. The most common methods used are based on the one hand on the pulse radar technique, and on the other hand, on the frequency modulated continuous wave radar technique (FMCW). When using frequency modulated continuous wave radar technique, the round trip time is determined indirectly via the transmission of a frequency modulated signal and formation of the difference between transmitted and received momentary frequency. Pulse radar technique, however, uses short microwave pulses, so-called bursts, and determines the direct time period between transmission and receipt of the pulses. From this time period, the distance between sensor and surface of the material, and therefore the level of material, can be derived.
From GB 2 094 091 A equivalent to DE 31 07 444 C2 and U.S. Pat. No. 4,521,778 A a high resolution coherent pulse radar system is known in which two high stability pulse series differing only slightly in their pulse repetition frequency are produced in a transmitter. Both series of pulses are converted into microwave pulse packets, one pulse series being processed into transmitting pulses and the other into scanning pulses for a time expansion process. Mixing of the scanning pulses and received echo pulses in a mixer results in the formation of an intermediate frequency signal and in the time expansion of echo pulses. The received signal bandwidth is thus reduced at the intermediate frequency stage. This method is also called sequential sampling or equivalent time sampling (ETS).
In DE 298 15 069 U1 a sampling circuitry for equivalent time sampling is disclosed in connection with a level measuring sensor using a guided microwave. This sampling circuitry comprises two oscillators, one of the oscillators is controlled in his frequency such that a measured frequency difference corresponds with a target value.
An apparatus for determining the filling level of a product in a container with a transmission unit which generates high frequency signals and emits them at a predetermined pulse repetition frequency in the direction of the surface of the filled product is known from DE 101 06 681 A1 equivalent to US 2002/0133303 A1. Here, the signals are reflected by the surface of the filled product and are received by the receiving unit. A delay circuit transforms the high frequency signals/reflected signals into low-frequency signals in accordance with a predetermined translation factor and with an evaluation unit which determines the filling level of the product in the container on the basis of delay time of the signals. The delay circuit includes: a transmission oscillator, a sampling oscillator, a digital sampling circuit, and a closed-loop/open-loop control unit.
In U.S. Pat. No. 6,680,690 B1 it is disclosed a radar level gauge for measuring a level of a surface of a product in a tank having a two-wire process control loop. It comprises an output circuitry coupled to the two-wire process control loop for setting in the loop a desired value of a loop current corresponding to the product level, a power supply circuitry coupled to the two-wire process control loop for receiving power from the loop and being a source of power for a microwave source, a microwave receiver, a measurement circuitry and the output circuitry and including a converter for transferring power from the loop to said power supply circuitry by means of feeding a first current from the loop to the power supply circuitry, a current generator included in said output circuitry for generating in parallel to said first current a second current in the loop for maintaining said loop current at a value corresponding to said product level and a sensing circuit for determining the value of said second current and having an output indicative of the value of said second current.
Finally, U.S. Pat. No. 5,672,975 discloses a two-wire level transmitter for use in a process application which measures the hight of a product in a tank. Here, an output circuitry coupled to the two-wire process control loop transmits information related to a product height over the loop.
A widely-used standard in measurement and control technology is the so-called two-wire control loop. In general, this means that both the supply of power to the apparatus as well as data communication, e.g. the output of a measured value, results via the same pair of wires. Common standards are the 4-20 mA two-wire standard, as well as various other standards, according to which the data communication of the apparatus results via signals digitally modulated onto the pair of wires. However, two-wire control loops are limited in providing sufficient energy for the measuring devices used in the field.
From U.S. Pat. No. 6,014,100, a two-wire radar sensor is known, the power consumption of which exceeds the above-mentioned value at least in one phase of the measurement cycle. In other phases of the measuring cycle, the power consumption lies below the limit. By means of buffering energy in phases of low power consumption, and supplying the energy stored in phases of high power consumption, a two-wire operation is enabled.
Measuring cycle means the time between determining two subsequent measurement results for the level of material. The measuring cycle can be, according to the above-mentioned prior art, subdivided into several phases of different power consumption: In a first measurement phase, in which the microwave pulses are transmitted and received, are dilated by the sampling process, and are represented as an echo profile of the measurement range, the power consumption is at a maximum. A second phase is characterized by a minimum power consumption, which is achieved by switching off the transmit/receipt circuit and maintaining an analyzing unit in an energy saving standby mode. In a third phase, during which the echo analyzing unit selects and analyzes the echo generated in the echo profile, the power consumption also clearly is below the maximum. The second and third phase can be interchanged in their order, or can be nested into each other.
The buffering of the electrical energy e.g. in capacitors during the second and third phase, in which the power consumption is low, enables for a power consumption lying clearly above the limit set by the two-wire control loop during the first phase. Latest developments in the field of radar level sensors aim at miniaturizing the electronic circuit, and thus, the entire apparatus. On the other hand, an improved signal sensitivity is desired, to also detect bulk material reflections within the container precisely and reliably, even if conditions are unfavorable or at large measuring distances.
Higher signal sensitivity can be achieved by measures as increasing the transmission performance, prolonging the time period of the transmission pulses, amplifying the microwave receipt signals by means of a low noise amplifier, and/or increasing the dilation factor. For the two-wire control operation of such an improve sensor, however, the problem arises that the power consumption will rise dramatically during the first phase of measurement. Moreover, the time period for carrying out a complete measurement within the first phase will increase due to the increased dilation of the echo profile and the broadening of the possible measurement range. By extension of the second phase, it could, however, be provided for the average power consumption not exceeding the limit set by the two-wire control, but for covering the comparatively high power consumption in the first measuring phase, large energy stores would be necessary, which cannot be united with circuit miniaturization. Shortening of the first measuring phase can be excluded, because according to prior art, always a complete echo profile of the whole measuring range has to be generated, starting with a reference echo in the close-up range up to the distance to be measured maximally. If a large measuring range and a high dilation factor are used, then, a comparatively long time period results, during which a high power consumption has to be covered without any interruptions.